Highly conductive carrier particles

ABSTRACT

ELECTRICALLY CONDUCTIVE CARRIER PARTICLES FOR USE IN APPLYING ELECTROSCOPIC TONER MATERIAL TO ELECTROSTATIC CHARGE PATTERNS ARE PREPARED BY OVERCOATING A MAGNETIC CORE MATERIAL, SUCH AS BY ELECTROLESS DEPOSITION OR ELECTROPLATING METHODS, WITH A THIN CONTINUOUS LAYER OF AN ELECTRICALLY CONDUCTING METAL HAVING A RESISTANCE TO AERIAL OXIDATION GREATER THAN THAT OF IRON. THESE CARRIER PARTICLES RETAIN THEIR ELECTRICAL PROPERTIES DURING REPEATED USAGE AND ARE HIGHLY ABRASION RESISTANT.

United States l atent C) F 3,736,257 HIGHLY CONDUCTIVE CARRIER PARTICLES Howard A. Miller, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y.

No Drawing. Continuation-impart of abandoned application Ser. No. 799,967, Feb. 17, 1969. This application Dec. 21, 1970, Ser. No. 100,299

Int. Cl. G03g 9/02 U.S. Cl. 25262.1 16 Claims ABSTRACT OF THE DISCLOSURE Electrically conductive carrier particles for use in applying electroscopic toner material to electrostatic charge patterns are prepared by overcoating a magnetic core material, such as by electroless deposition or electroplating methods, with a thin continuous layer of an electrically conducting metal having a resistance to aerial oxidation greater than that of iron. These carrier particles retain their electrical properties during repeated usage and are highly abrasion resistant.

This application is a continuation-in-part application of Miller, U.S. patent application Ser. No. 799,967, filed Feb. 17, 1969, now abandoned.

This invention relates to electrography, and more particularly, to magnetically attractable carrier particles useful in the magnetic-brush type development of electrostatic latent images.

Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, US. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others. Generally, these processes have in common the steps of employing a normally insulating photoconductive element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. The electrostatic latent image is then rendered visible by a development step in which the charged surface of the photoconductive element is brought into contact with a suitable developer mix.

One method for applying the developer mix is by the well-known magnetic brush process. Such a process can utilize apparatus of the type described, for example, in U.S. Pat. No. 3,003,462 and often comprises a nonmagnetic rotatably mounted cylinder having fixed magnetic means mounted inside. The cylinder is arranged to rotate so that part of the surface is immersed in or otherwise contacted with a supply of developer mix. The granular mass comprising the developer mix is magnetically attracted to the surface of the cylinder. As the developer mix comes within the influence of the field generated by the magnetic means within the cylinder, the particles thereof arrange themselves in bristle-like formations resembling a brush. The bristle formations of developer mix tend to conform to the lines of magnetic flux, standing erect in the vicinity of the poles and lying substantially flat when said mix is outside the environment of the magnetic poles. Within one revolution the continually rotating tube picks up developer mix from a supply source and returns part or all of this material to the supply. This mode of operation assures that fresh mix is always available to the copy sheet surface at its point of contact with the brush. In a typical rotational cycle, the roller performs the successive steps of developer-mix pickup, brush formation, brush contact with the photoconductive element, brush collapse and finally mix release.

In magnetic-brush development of electrostatic images,

Patented May 29, 1973 the developer is commonly a triboelectric mixture of fine toner powder comprised of dyed or pigmented thermoplastic resin with coarser carrier particles of a soft magnetic material such as ground chemical iron (iron filings), reduced iron oxide particles or the like. The conductivity of the ferromagnetic carrier particles whichform the bristles of a magnetic brush gives some advantage over other modes of development. The conductivity of the ferromagnetic fibers or bristles provides the effect of a development electrode having a very close spacing to the surface of the electrophotographic element being developed. By virtue of this development electrode effect it is to some extent possible to develop part of the tones in pictures and solid blacks as well as line copy. This ability to obtain solid area development with magnetic brush developing makes this mode of developing advantageous where it is desired to copy materials other than simple line copy.

However, most currently available ferromagnetic carrier particles have an electrical resistance which is too high to produce good quality solid area development. The various commercial carrier particles generally lack adequate conductivity because of the presence of an insulating surface layer of iron oxide, grease or other contaminants. Efforts to remove such surface contaminants often result in particles which have an even higher electrical resistivity. For example, washing or solvent treatment of iron carrier particles in an effort to remove contaminants merely exposes the surface of the underlying iron to aerial oxidation. The new layer of oxide often has far greater resistivity than the original contaminants. Such an oxide coating can be removed; however, special after-treatment and precaution in storage and handling are required in order to avoid any further oxidation.

Accordingly, there is a need for a method of converting commercially available carrier powders into stable car-rier particles having sulficient surface conductivity to promote good, solid area development.

It is, therefore, an object of this invention to provide novel highly conductive magnetic carrier particles having good magnetic response and having good stability under ordinary conditions of storage and use.

It is another object of this invention to provide a new, simple, reliable and economic means for the production of stable, highly conductive carrier particles from a variety of commercially available iron and iron-alloy powders.

A further object of this invention is to provide new developing compositions for use in solid area or continuous tone development of electrostatic charge patterns by cascade or magnetic brush techniques.

Still another object is to provide a new process for solid area development of electrostatic latent images.

These and other objects and advantages are accomplished in accordance with this invention which features conventional carrier materials, for example iron and ironalloy powders, bearing a thin layer of an electrically conducting metal resistant to aerial oxidation, said layer coated thereover by a controllable, self-perpetuating reduction of dissolved metal ion. As used in the present specification coating by controllable, self-perpetuating reduction of dissolved metal ion is understood to include coating methods such as, for example, electroless (or catalytic chemical) plating and electroplating. Coating methods such as vacuum evaporation, cathode sputtering or frictional methods such as ball-milling which do not involve dissolved metal ions would not be included by this term. Other coating methods such as simple chemical displacement coatiug processes which are self-limiting or methods such as spontaneous, noncatalytic mirror-plating processes would also not be included by this term.

The core materials which can be suitably overcoated in accordance with this invention are materials which are relatively strongly attracted to a magnet; suitable magnetic materials would include iron in such forms as reduced iron oxide bits, iron filings and the like; also useful are ferromagnetic iron alloys such as those containing iron, nickel and/or cobalt. Such ferromagnetic materials are used as a core in accordance with this invention over which core is coated a thin, continuous, adherent layer of a highly conducting metal which is resistant to aerial oxidation of the type which impairs electrical conductivity. The magnetic core can vary in size and shape with useful results being obtained with core sizes of from about 120014. to about 40p. average diameter. Particularly useful results are obtained with core materials of from about 600a to about 60 average diameter. The size of the core particles used will, of course, depend upon several factors such as the type of images ultimately developed, desired thickness of the metal coating, etc. The phrase average diameter as used herein is not meant to imply that only perfectly uniformly dimensioned particles can be used. This phrase is used to generally refer to the average thickness of particles when measured along several axes. Average diameter or particle size also refers generally to the approximate size of the openings in a standard sieve series which will just retain or just pass a given particle.

The materials useful for coating or plating onto the core are typically metals which are substantially more resistant to surface oxidation than iron and iron-alloys. Suitable coating materials having a resistance to aerial oxidation greater than that of iron include those metals in Groups VIa, VIII, Ib and III: of the Periodic Table (Cotton and Wilkinson, Advanced Inorganic Chemistry, 1962, page 30). Particularly useful metals are cadmium, chromium, copper, gold, nickel, silver, zinc, and the platinum group elements which include ruthenium, rhodium, palladium, osmium, iridium and platinum as well as mixtures or alloys of any of these. Most of these metals are more electronegative than the iron starting material which property is advantageous in certain coating procedures. With other coating procedures, the metals more electropositive than iron can be useful such as chromium, zinc and cadmium.

The useful conducting metals all have a greater corrosion resistance or resistance to aerial oxidation than does iron. The terms corrosion resistance or resistance to aerial oxidation all have reference to the ability of a metal to withstand oxidative-type corrosion which impairs electrical conductivity. In general, the type of corrosion which should be avoided is continuous aerial oxidation or rapid aerial oxidation which substantially reduces the electrical conductivity of a metal. In particular, these terms have reference to corrosion induced by exposure to air, carbon dioxide, water vapor, ozone, etc., and do not have particular reference to the chemical attack of solutions of strong acids or bases, etc.

The carriers of this invention all have thin, continuous adherent coatings of an electrically conducting metal. In general, the conducting coating or layer is from about 51. to about 100 in average thickness, with a preferred thickness not exceeding about 50 and with a most economical thickness not exceeding about 20 11.. The shape of the starting core material will, of course affect the average thickness of the conducting layer. Very smooth starting materials can be made highly conductive by this technique using only a very thin conducting layer. On the other hand, starting materials which are rough, pitted, fissured, etc., generally require a thicker conducting overcoat layer.

The conductive coating characterizing the carrier particles of the present invention is a highly coherent, abrasion resistant, continuous electrically conducting outerlayer. Numerous methods can be used to provide metal layers having sufficient adherence to the magnetic core so that th r ulta t Far i r r ivles may he us s is s m ype of electrophotographic developer. However, where long-life carriers having superior abrasion and corrosion resistance are desired, it has been found that layers of electrically conducting metal which are coated over the magnetic core by a controllable, self-perpetuating reduction of dissolved metal ion provide substantially superior coatings which exhibit better adhesion and cohesion, and as a consequence much greater resistance to corrosion and abrasion than coatings applied by various other methods. Electroplatin-g and electroless methods of catalytic chemical deposition of nickel, cobalt, palladium and the like, are among the useful processes for making the carrier particles of the present invention with electroless deposition methods being particularly preferred. Iron and many other metal particles can be electroplated directly, while certain other metal particles as well as nonmetallic particles may require formation of a palladium or other catalyst layer first before the final layer can be added.

A surprising feature of the present invention is the discovery that carrier particles having tightly adherent conductive coatings may be prepared by the present invention by coating an intermediate metallic layer over the magnetic core using a method such as, for example, vacuum deposition or simple chemical displacement, and then overcoating this intermediate layer with a layer applied according to the present invention. The surprising feature resulting is that although an intermediate layer applied by vacuum deposition or simple chemical displacement is substantially less abrasion resistant than a similar coating applied by electroless deposition or electroplating, a carrier particle having such an intermediate layer overcoated with a layer according to the present invention can have substantially the same superior abrasion resistance as a carrier particle having an overlayer or layers applied only in accordance with the present invention, i.e., applied without using known carrier coating methods such as chemical displacement or vacuum deposition. This aspect of the invention is illustrated in Ex. 8 of the present specification.

The continuous layers formed in accordance with this invention are very adherent to the core material. In view of the fact that the present conductive layers are continuous and not simply formed of many beads of metal on a core, the present layers show virtually no tendency to flake off or become separated from the core during repeated use. In fact, a test of carrier particles which have an overcoat of conducting metal applied in a conventional manner shows such materials to be highly prone to degradation during use. In particular, with continued use of only short duration the metal deposited on such conventionally coated particles begins to flake or chip off leaving the core uncoated. As a consequence of this flaking, the resistivity of carriers so coated goes up rapidly with continued use. However, when carrier particles are overcoated with a thin, continuous adherent layer of metal in accordance with this invention, the outer layer is not subject to such objectionable flaking. Consequently, the present carrier particles retain their high conductivity or low resistivity even during repeated usage. In view of the fact that the electrical properties of the particles of this invention are not substantially altered during prolonged usage, these particles have a long useful life and do not have to be replaced very often.

In general, the electrical resistance of the carrier particles of this invention is less than about ohms. However, for best results in solid area or continuous tone development, it is preferred that the carriers have a re sistance of less than about 10 ohms. 0t. course, for certain applications it may be desirable to have a resistance value of greater than 100 ohms. For purposes of comparison, the resistance of the various carrier particles is measured in a standard electrical resistance test. This test is conducted each time using a 15 gram quantity of carrier material. A cylindrically-shaped bar magnet having a circular end of about 6.25 square centimeters in area is used to attract the carrier and hold t. n he. form of a brush.

After formation of the brush, the bar magnet is positioned with the brush-carrying end approximately parallel to and about 0.5 cm. from a burnished copper plate. The resistance of the particles in the magnetic brush is then measured between the magnet and the copper plate.

Electroscopic developer compositions can be prepared by mixing the present highly electrically conductive carriers with a suitable electroscopic toner material. In general, useful developers are comprised of from about 90 to about 99% by weight of carrier and from about to about 1% by weight of toner. The toner used with the carrier particles of this invention can be selected from a wide variety of materials to give desired physical properties to the developed image and the proper triboelectric relationship to match the carrier particles used. Generally, any of the toner powders known in the art are suitable for mixing with the carrier particles of this invention to form a developer composition. When the toner powder selected is utilized with ferromagnetic carrier particles in a magnetic-brush development arrangement, the toner clings to the carrier by triboeletcric attraction. The carrier particles acquire a charge of one polarity and the toner acquires a charge of the opposite polarity. Thus, if the carrier is mixed with a resin toner which is higher in the triboelectric series, the toner normally acquires a positive charge and the carrier a negative charge.

Toner powders suitable for use in this invention are typically prepared by finely grinding a resinous material and mixing with a coloring material such as a pigment or a dye. The mixture is then ball milled for several hours and heated so that the resin flows and encases the coloring material. The mass is cooled, broken into small chunks and finely ground again. After this procedure the toner powder particles usually range in diameter from about 0.5 to about 25,17, with an average size of about 2 to about p.

The resin material used in preparing the toner can be selected from a wide variety of materials, including natural resins, modified natural resins and synthetic resins. Exemplary of useful natural resins are balsam resins, colophony and shellac. Exemplary of suitable modified natural resins are colophony-modified phenol resins and other resins listed below with a large proportion of colophony. Suitable synthetic resins are all synthetic resins known to be useful for toner purposes, for example, polymers, such as vinyl polmers including polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate, polyvinyl acetals, polyvinyl ether and polyacrylic and polymethacrylic esters; polystyrene and substituted polystyrenes or polycondensates, e.g. polyesters, such as phthalate resin, terephthalic and isophthalic polyesters, maleinate resin and colophony-mixed esters of higher alcohols phenolformaldehyde resins, including colophony-modified phenol-formaldehyde condensates, aldehyde resins, ketone resins, polyamides and polyadducts, e.g. polyurethanes. Moreover, polyolefins such as various polyethylenes, polypropylenes, polyisobutylenes and chlorinated rubber are suitable. Additional toner materials which are useful are disclosed in the following US. Patents: 2,917,460; Re. 25,136; 2,788,288; 2,638,416; 2,618,552 and 2,659,670.

Color material can be incorporated into toners to render electrostatic images toned therewith more distinct or visible. The coloring material additives useful in suitable toners are preferably dyestuffs and colored pigments. These materials serve to color the toner and thus render it more visible. In addition, they sometimes affect, in known manner, the polarity of the toner. In principle, virtually all of the compounds mentioned in the Color Index, vols. I and II, second edition, 1956, can be used as colorants. Included among the vast number of suitable colorants would be such materials as Nigrosin Spirit soluble (C.I. 50415), Hansa Yellow G (CI. 11680), Chromogen Black ETOO (C.I. 14645), Rhodamine B (CI. 45170), Solvent Black 3 (CI. 26150), Fuchsine N (C.I. 42510), C.I. Basic Blue 9 (0.1. 52015), etc.

The following examples are included for a further understanding of the invention. Throughout the examples, the mesh screen sizes given refer to US. Standard Sieve Series.

EXAMPLE 1 A 500 g. quantity of commercial iron powder having a resistance as measured in the standard tests referred to above of 2400 ohms and having a particle size such that it will pass through a 60 mesh screen and be retained by a 120 mesh screen is washed by agitation in 1500 m1. of a solution of 1% Alconox (a synthetic detergent) in tap water for three minutes, followed by decantation rinsing in 5 changes of cold water. Excess water is removed by suction filtration and the clean iron powder is electroplated with a thin continuous layer of zinc in a plating bath having the following composition:

G. Zinc sulfate ZnSO -7H O 300 Sodium chloride NaCl 15 Aluminum sulfate Al (SO 20 Boric acid H BO 20 DeXtrin 10 Water to make 1 l.

A zinc-plated steel grid cathode is positioned at the bottom of the bath such that the iron powder being plated is maintained in conductive contact with the cathode by gravity. The anode used is also zinc. The powder is stirred continuously at a very slow rate to prevent sintering or caking. The plating current is maintained at 100 milliamperes per calculated square inch of iron surface for a total plating time of twenty minutes at about 20 C. This gives a zinc coating having an average thickness of from about 4 to 5 microns. The resulting zinc-plated iron powder is then washed several times with cold water, rinsed three times with anhydrous isopropanol, spread out and dried on a glass plate with continuous agitation. The resistance of the plated powder as measured in the standard test is 7 ohms. A 100 g. portion of the zinc plated iron is then compared with an equal quantity of the starting material and with a similar amount of starting material which has been subjected to the detergent wash, rinsing and drying steps without any subsequent plating step. Magnetic-brush developers are prepared by mixing each of the three carrier materials with separate 3 g. portions of black toner material comprising nigrosine colorant in a polystyrene resin binder. The resultant developers are then each used in a hand-held magnetic brush to manually develop a negatively charged electrostatic latent image carried on an organic photoconductor-containing electrophotographic element. All three developers produce an image; however, the zinc-coated carrier powder gives greatly improved solid area development, better density and better overall image quality than do the other two carrier materials.

EXAMPLE 2 atabout C. in a bath having the following composition:

Nickel(ous) chloride NiCl -6H O' g 67.5 Sodium citrate Na C H O -2H O g 123.0 Ammonium chloride NH Cl g 75.0 Ammonia solution (28%) m1 150.0 Sodium hypophosphite NaH Po -H o g 16.5

Water to make 1500.0 ml.

The nickel-plated powder is then washed in six changes of cold water, rinsed three times with ethanol, filtered on a sintered glass funnel to remove most of the alcohol and spread out on clean paper to dry. This procedure produces a nickel layer having an average thickness of about 1 micron. After coating, the resistance of the dry powder is 0.5 ohm as measured in the standard test. Eight hundred grams of the nickel-clad iron and an equal quantity of the uncoated particulate iron starting material are then used to make two developer compositions by mixing each with a 24 g. portion of a black polystyrene toner powder having an average particle size of microns. The two developers are tested as in Example 1 only using a commercial design motor-driven magnetic brush apparatus. This apparatus is of the general type described in US. Pat. No. 3,003,462 and comprises a motor-driven, nonmagnetic rotatably mounted cylinder having fixed magnetic means mounted inside. This cylinder is arranged such that it will rotate with part of its surface in contact with a reservoir of developer composition. The control developer gives poor image quality prints and nonuniform solid areas. The developer made with the nickel coated iron yields images of greatly improved overall quality with good image density and solid areas.

EXAMPLE 3 The procedure of Example 2 is repeated using a commercial electrolytic iron powder having a particle size such that it will pass through a 60 mesh screen and be retained by a 120 mesh screen. The resultant nickel plating has an average thickness of about 0.85 to 0.9 micron. The initial resistance of the material is 600,000 ohms. After plating, the resistance as measured in the standard test referred to above is reduced to 0.7 ohm. The starting material and the nickel plated carrier material are used to form developers and tested as in Example 2. The developer composition containing the nickel-plated carrier shows improved image quality comparable to that obtained with the nickel-plated spherical iron powder of Example 2. In addition, it is possible to increase the processing rate (the rate of relative movement of the electrophotographic element in relation to the magnetic brush) to approximately three times that possible with the starting iron carrier before any serious decrease in the degree of development and uniformity of the solidarea development is encountered.

EXAMPLE 4 The plated carriers of Examples 1 and 2 are compared with the corresponding uncoated starting materials in order to determine the tendency of each to deposit scum on the surface of an electrophotographic element. The various carriers are tested on an apparatus of the type described in Example 2. All four carrier materials are first tested alone in the apparatus and then each of the carriers is tested in admixture with 3% by weight of a colored toner material as in Example 1. In each test, the cylinder is run at a linear speed of 22 ft./min. and each magnetic brush formed is run in contact with an electrophotographic element for a period of 15 minutes. When tested both with and without toner, the two uncoated iron carriers (controls 1 and 2, respectively) leave a very tenacious deposit of black material which is apparently a mixture of dirt and small pieces of iron chipped 01f of the carriers. Control 1 appears to give somewhat more scum than does Control 2. However, the zinc and nickel coated carriers leave no visible deposit when tested as above. The four carriers are again tested using 5% by weight of the toner of Example 1. The control carriers again show considerable scumming; whereas, the coated carriers show to scumming tendency.

EXAMPLE 5 The nickel-clad iron carrier of Example 3 is used to prepare a developing composition by mixing 96% by 8 Weight of the coated iron and 4% by weight of the black toner composition of Example 1. The developer composition is then used in a scum test machine as referred to in Example 4 for a period of 8 hours daily for 25 days. No change in appearance of the developer, in the tendency of the toner to be thrown off of the carrier or in the developing properties is noted. The test is then repeated again using the control developer of Example 3 and after about three days of running the apparatus, the toner begins to be thrown off such that it creates a serious problem.

EXAMPLE 6 A two-kilogram quantity of reduced iron powder (Glidden Iron No. 388, Glidden-Durkee Div. SCM Corp.) having a particle size such that it will pass through an mesh screen and be retained by a mesh screen, is added to 1500 ml. of a 5% hydrochloric acid solution, agitated and rinsed by decantation with water at about 5 C. The damp powder is then added to 2500 ml. of an electroless copper plating bath at a temperature of about 20 C. and having the following composition:

Solution A G. Sodium potassium tartrate tetrahydrate 200 Sodium hydroxide 60 Copper sulfate tetrahydrate 40 Sodium carbonate monohydrate 40 Ethylenediamine tetracetic acid disodium salt 6 Water to make 1 liter.

Solution B Ml. Formaldehyde solution (37%) Water to make 1 liter.

Equal parts of Solutions A and B are mixed together and the pH is adjusted to about 11.8 with addition of sodium hydroxide or HCl as required. The powder is continuously agitated in the bath for about 10 minutes during which time the gray iron surface is covered with a bright orange layer of copper. The plating bath is then decanted and the wet powder is rinsed five times by decantation with water at about 5 C. and then rinsed three times with denatured ethanol. The damp powder is dried on a glass plate at room temperature with continuous mixing. The dry powder is then tested in the standard resistance test and found to have a resistance value of about 0.8 ohm. The copper plating shows good abrasion resistance after 16 minutes of continuous agitation of 24 g. of carrier in a 4 02. glass jar. After agitation, the resistance value is not greatly changed, and the powder still appears orange in color. Next, 100 g. of the coated carrier particles are mixed with 4% by weight of a toner powder comprised of nigrosine and a styrene-containing polymer. The toner powder has an average particle size of about 6 to 10 microns. The resultant developer composition is then used to develop negative electrostatic latent images carried on a photoconductive element. Good quality solid-area developed images are obtained.

EXAMPLE 7 A 1 kilogram quantity of reduced iron powder having a particle size such that will pass through an 80 mesh screen and be retained by a 150 mesh screen is washed three times with 1000 cc. of denatured ethanol and dried at room temperature with continuous mixing. A 300 g. quantity is retained as the control for this experiment. A second 300 g. quantity is copper plated by adding it over a period of 15 seconds to the vortex of 500 ml. of 2% copper sulfate solution at about 21 C. in a high speed mechanical blender which is running at suificient speed to maintain a uniform dispersion of the powder without settling. After two minutes treatment, the copper plated powder is allowed to settle, the supernatant liquid is. dra ned 01f. and the Wet powder s rinsed with our changes of cold water with settling and draining after each addition of water. The wet powder is rinsed four times with denatured ethanol and the surplus alcohol is removed after the last rinse by suction filtration. The damp powder is then air dried with continuous mixing on a glass plate to yield about 294 g. of orange-brown powder. A third 300 g. quantity of iron is nickel-plated by the electroless plating procedure of Example 2 above. Next the three samples are tested for electrical properties and for adherence of the copper and nickel platings. This test consists of placing 24 g. quantities of the three powders in separate 4-ounce jars which are sealed and shaken vigorously by hand. Resistance readings of the powder are made, in accordance with the standard resistance test referred to above, before shaking the powder and after 1, 2, 4, 8, and 16 minutes agitation. The results of these resistance tests are shown in Table I below:

TABLE I Resistance (ohms) at- After only one minute of agitation, and increasingly thereafter, the coppered iron develops an appreciable amount of very finely divided material which leaves a brown residue when the powder is allowed to cascade down an inclined sheet of rough white paper. Neither of the other carriers, the control iron or the nickeled iron, show any accumulation of such fine particles. Next, the three carriers are used to prepare developer compositions consisting of 100 g. of the carrier materials mixed with separate 5 g. quantities of a black polystyrene base toner powder of 7 microns average particle size. The respective developer compositions are tum-bled in 8-ounce glass jars for 20 minutes to simulate the agitation received during preparation and brief use of a suitable triboelectric developer. The three developers are then used in a manual magnetic brush to develop negative electrostatic latent images carried on an organic photoconductive film. The developer repared from the nickeled carrier gives more complete solid area development and better image quality than do the other two developers. The nickel-containing developer also produces generally cleaner background and is less sensitive to changes in bias voltage. The overall quantity of the prints made with the coppered carrier is no better than that of the prints made with the untreated iron carrier. In addition, on examination of the prints with a magnifying lens, numerous bits of unwanted copper fines in the background and around the edges of the developed image formed using the coppered carrier. This example thus shows the importance of having a conducting coating which is in the form of a continuous, strongly adherent layer.

EXAMPLE 8 The following four magnetic powders are used to prepare carrier particles in accordance with this invention. In the following procedures 100 gram quantities of each of the powders listed below are used and these powders have an average particle size such that they will pass through an 80 mesh screen and be retained by a 120 mesh screen. Before coating, each of the powders is measured for electrical resistance in the standard resistance test referred to above and found to have a resistance of greater than 1 megohm.

(l) Spherical beads of iron powder (Federal Mogul Co.) made by spraying molten pure iron into cold water to provide an insulating oxide powder coating.

(2) Ferroxcube A powder which is a soft ferrite material composed of 48% MnO'-Fe O .52% ZnO-Fe O (3) Ferromagnetic chromous oxide which is then granulated with 6% of a urea formaldehyde prepolymer dispersion (Rohm and Haas Uformite F-240). After granulation to proper size through an mesh screen,

the dried powder is thermoset by infrared radiation. (4) A carrier powder comprised of by weight of colloidally fine carbonyl iron powder dispersed in 15% polyvinyl acetate as described in copending US. patent application Ser. No. 562,497, filed July 5, 1966, now abandoned.

Each of the four insulating core materials is prepared for application of an oxidation-resistant, electrically conducting nickel outerlayer by immersing and stirring gently in 125 ml. of a 2% stannous chloride solution containing 0.5 hydrochloric acid at 30 C. After two min utes treatment, the solution is removed by decantation, the powder rinsed with three changes of tap water at 30 C. and then mixed with 125 ml. of 0.25% palladium chloride solution containing 0.5% hydrochloric acid at 30 C. Stirring is continued for two minutes followed by five decantation rinses with 200 ml. portions of water at 30 C. The wet particles carrying a catalytic layer of pa ladium as laid down in the above treatment are nickel plated by electroless deposition using the procedure described in Example 2. After nickel-coating the above core materials, a 15 gram quantity of each was tested in the standard resistance test referred to above and found to have a resistance value of 2 ohms or less. Next, a magnetic brush development composition is prepared with each of the above carriers by using 96% by weight of the carrier with 4% by weight of a black polystyrene based toner powder having an average particle size of about 10 microns. An organic photoconductive film is subjected to negative corona discharge, imagewise exposed and developed with a hand magnetic brush using one of the above developer compositions. A developed image is transferred to white bond paper with the aid of a negative corona. This procedure is repeated for each of the developer compositions containing the coated carriers and is then again repeated using the four uncoated insulating core materials as the carriers. The developers containing the carriers of this invention produce images having good solid area development and excellent general overall image quality; whereas, the control developers produced images of very low density with nonuniform development in the solid areas and generally lower overall image quality.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected Within the spirit and scope of the invention.

I claim:

1. In a developer composition for use in the development of electrostatic charge patterns and comprising a physical mixture of magnetically attractable carrier particles and smaller electroscopic toner particles, the improvement wherein said carrier particles are comprised of a core of ferromagnetic material having electroplated thereon a separate, thin, continuous outermost layer of an electrically conducting metal (a) having a resistance to aerial oxidation greater than that of iron and (b) selected from the group consisting of nickel, cobalt, copper, Group IIb metals, and Group VIa metals, said carrier particles having an electrical resistance of less than about 100 ohms.

2. A developer composition as in claim 1 wherein the overcoated layer of conducting metal has a thickness of from about to about 100 microns.

3. A developer composition as in claim 1 wherein the core material is selected from the group consisting of iron and iron alloys.

4. In a developer composition for use in the development of electrostatic charge patterns and comprising a physical mixture of magnetically attractable carrier particles and smaller electroscopic toner particles, the improvement wherein said carrier particles are comprised of a core of ferromagnetic material having electrolessly plated thereon a separate, thin, continuous outermost layer of an electrically conducting metal (a) having a resistance to aerial oxidation greater than that of iron and (b) selected from the group consisting of nickel, cobalt, copper, Group IIb metals, and Group VIa metals, said carrier particles having an electrical resistance of less than about 100 ohms.

5. A developer composition as in claim 4 wherein the overcoated layer of conducting metal has a thickness of from about & to about 100 microns.

6. A developer comprising as in claim 4 wherein the core material is selected from the group consisting of iron and iron alloys.

7. In a developer composition for use in the development of electrostatic charge patterns and comprising a physical mixture of magnetically attractable carrier particles and smaller electroscopic toner particles, the improvement wherein said carrier particles are comprised of a core of a material selected from the group consisting of iron and iron alloys, said core having electroplated thereon a thin, continuous outermost layer of an electrically conducting metal selected from the group consisting of nickel, cobalt, copper, Group Wu, and Group IIb metals.

8. A developer composition as in claim 7 wherein the conducting metal is selected from the group consisting of cadmium, chromium, cobalt, copper, nickel, zinc and alloys thereof.

9. A developer composition as in claim 7 wherein the conducting metal is nickel.

10. A developer composition as in claim 7 wherein the average diameter of the core is from about 40 to about 1200 microns.

11. In a developer composition for use in the development of electrostatic charge patterns and comprising a. physical mixture of magnetically attractable carrier particles and smaller electroscopic toner particles, the improvement wherein said carrier particles are comprised of a core of a material selected from the group consisting of iron and iron alloys, said core having electrolessly plated thereon a thin, continuous outermost layer of an electrically conducting metal selected from the group consisting of nickel, cobalt and copper, Group Vla, and Group Hb metals.

12. A developer composition as in claim 11 wherein the conducting metal is selected from the group consisting of cadmium, chromium, copper, cobalt, nickel, zinc and alloys thereof.

13. A developer composition as in claim 11 wherein the conducting metal is nickel.

14. A developer composition as in claim 11 wherein the average diameter of the core is from about 40 to about 1200 microns.

15. In a developer composition for use in the development of electrostatic charge patterns and comprising a physical mixture of from about to about 99% by weight of magnetically attractable carrier particles and from about 1 to 10% by weight of smaller electroscopic toner particles, the improvement wherein said carrier particles are comprised of a core having an average diameter of from about 40 to about 1200 microns of a material selected from the group consisting of iron and iron alloy, said core having electrolessly plated thereon a thin, continuous, outermost overcoat of nickel, said particles having an electrical resistance of less than 100 ohms and said particles being characterized in that the electrical properties are not substantially altered with prolonged usa e.

1 6. In a developer composition for use in the development of electrostatic charge patterns and comprising a physical mixture of from about 90 to about 99% by weight of magnetically attractable carrier particles and from about 1 to 10% by weight of smaller electroscopic toner particles, the improvement wherein said carrier particles are comprised of a core having an average diameter of from about 40 to about 1200 microns of a material selected from the group consisting of iron and iron alloys, said core having electroplated thereon a thin, continuous, outermost overcoat of nickel, said particles having an electrical resistance of less than 100 ohms and said particles being characterized in that the electrical properties are not substantially altered with prolonged usage.

References Cited UNITED STATES PATENTS 3,166,444 1/ 1965 Ehren 117-49 3,054,751 9/ 1962 Blake et a1. 117-100 3,121,642 2/ 1964 Bishop 117-100 2,890,968 6/ 1959 Graime 252-621 2,874,063 2/1959 Greig 117-17.5 3,607,750 9/ 1971 Rarey 252-621 2,809,731 10/ 1957 Rau 11.7-234 FOREIGN PATENTS 572,459 3/1959 Canada 25 2-62.1 670,813 4/ 1952 Great Britain ll7100 817,698 8/1959 Great Britain 117-100 1,499,641 10/ 1967 France 252-621 OTHER REFERENCES Safranek, Symposium on Electroless Nickel Plating; ASTM Special Technical Publication No. 265; 1959, pp. 41-49.

J. TRAVIS BROWN, Primary Examiner J. P. BRAMMER, Assistant Examiner US. Cl. X.R. 11717.5, 100 

